Managing a fleet of vehicles requires procuring and replacing capital equipment like buses or planes that provide transportation services. Replacement costs exhibit economies of scale while maintenance costs exhibit dis-economies of scale. This paper makes three contributions. First, it develops the first formal models for determining optimal steady-state fleei designs. Second, it gives a well-defined and consistent definition of optimality for steady-state fleet designs. Third, it characterizes optimal steady-state fleet designs by showing that all replacement groups must be equally sized. This result makes computing optimal steady-state fleet designs possible since the computational effort is independent of the size of the fleet.